Jure Robic, a long-distance bicyclist who won the grueling Race Across America five times and whose seemingly endless, sleep-eschewing stamina tested the limits of human endurance, died during a training ride on Friday when he collided with a car on a mountain road in Plavski Rovt, Slovenia, near his home in Jesenice. He was 45.

Robic once rode 518.7 miles in 24 hours, a world record.

Primoz Kalisnik, a Slovene journalist and a friend of Robic, said that the driver of the car, a 55-year-old local man who was not hurt, was not at fault, and that Robic, who was going downhill on a mountain bike, may have been traveling as fast as 50 miles per hour on a narrow, winding stretch of unpaved road where it was impossible to see around the next bend. He was training for next month’s Crocodile Trophy mountain bike race in Australia, Kalisnik said.

Even in the circumscribed world of ultra-endurance athletes, Robic (his full name is pronounced YUR-eh ROH-bich) was known for his willingness, or his ability — or both — to push his body to extremes of fatigue. Compared by other riders to a machine and known to friends as Animal (a seeming contradiction that nonetheless made sense), he once rode 518.7 miles in 24 hours, a world record.

One occasional feature of his training regimen, which included daily rides or other workouts stretching between 6 and 10 hours, was a 48-hour period without sleep: a 24-hour ride followed by a 12-hour break followed by a 12-hour workout. Play, a magazine about sports that appeared in The New York Times, reported in 2006 that Robic rode 28,000 miles — more than the circumference of the Earth — every year.

His five victories in the Race Across America, an approximately 3,000-mile transcontinental ride that has been held annually since 1982, are unequaled. (The current course extends from Oceanside, Calif., to Annapolis, Md.)

Unlike the Tour de France, the Race Across America is not a stage race; once it begins, there is no respite for riders until they give up or cross the finish line, so determining when and how long to sleep is the event’s primary strategic element. The winner generally sleeps less than two hours out of 24 and finishes in less than nine days (although Robic’s winning time this past June was a relatively lethargic 9 days 46 minutes).

In 2005, Robic won the race and two weeks later won Le Tour Direct, a 2,500-mile European version with a course derived from Tour de France routes that included 140,000 feet of climbing — almost the equivalent of starting at sea level and ascending Mt. Everest five times. His time was 7 days 19 hours.

Robic became accustomed to both the physical and mental stress that pushing himself to extremes brought on. In the later stages of long-distance races, feet swell as much as two sizes and thumb nerves go dull from the pressure of hands on handlebars. Robic told Daniel Coyle, the Play magazine reporter, that for weeks after the Race Across America, he had to use two hands to turn a key.

“Don’t even ask about the derrière,” Mr. Coyle wrote. “When I did, Robic pantomimed placing a gun in his mouth and pulling the trigger.”

The mental anguish may be worse. As each race went on, Robic’s temper grew shorter and occasionally exploded. He was prone to hallucinations. More than once he leapt off his bicycle to do battle with threatening attackers who turned out to be mailboxes. Once he imagined he was being pursued by men with black beards on horseback — mujahedeen, he explained to his support team, who encouraged him to ride faster and keep ahead of them.

In 2003, the first time Robic entered the Race Across America, finishing second, Kalisnik volunteered to work on his team and was stunned by the changes the event wrought in Robic’s demeanor.

“We were just a group of guys helping a friend,” Kalisnik said. “We discovered someone we were absolutely afraid of.”

Robic knew this about himself.

“In race, everything inside me comes out,” he said. “Good, bad, everything. My mind, it begins to do things on its own. I do not like it, but this is the way I must go to win the race.”

Robic was born in Jesenice on April 10, 1965. From 1988 to 1994, he was a member of the Slovene national cycling team, and until recently he was a soldier in the Slovene army, a member of its athletic corps, which allowed him to train full time. (Among other methods employed during races to penetrate Robic’s numbing exhaustion and motivate him, his crew members, riding in a van behind him, sometimes blared Slovene military music through a loudspeaker.)

Robic’s marriage ended in divorce; he is survived by a half brother and a son. A brother, Saso, a former professional skier, committed suicide earlier this year.

“He was two personalities within one body,” said Kalisnik, who called his friend the most popular athlete in Slovenia. “One was very polite and nice when he was not on the bike. During races, he was absolutely the most unpleasant person you could imagine.”